Crossing the Palmera bridge, south of Rocha and noticed a bright sun-ball moving silently over some trees to my left. The ball was yellow in color and varied in its brightness (one second it was dull, the other it was blinding). I stopped at the end of the bridge and watched it for a little and saw it change direction towards the river. I then quickly lost sight of it. It was the size of a large helicopter. It was not the sun, it was too late and moved.